I just built a new garage and I am finishing the inside. I'm planning on putting up white metal pro rib sheets on the ceiling from Menards. Do I need to put plastic up on the bottom of the trusses before I install the metal? I'm going to blow in insulation in the attic.
Metal Ceiling Vapor Barrier
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Musicinabottle
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actually NO!
The reason being that if moisture gets in behind the metal, the plastic will trap it, with no real pathway to escape. If you're worried about a vapor barrier, I'd mastic with butyl bead, each joint. If your area above the ceiling is vented, then I wouldn't be concerned with a vapor barrier, because the metal, if properly installed is the vapor barrier itself.
The reason being that if moisture gets in behind the metal, the plastic will trap it, with no real pathway to escape. If you're worried about a vapor barrier, I'd mastic with butyl bead, each joint. If your area above the ceiling is vented, then I wouldn't be concerned with a vapor barrier, because the metal, if properly installed is the vapor barrier itself.
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R5P7Duster
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I've heard mixed things myself. I'd hate for my ceiling to start rusting from water sitting on it.
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rburke65
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NE Ohio here and I did plastic sheeting above my metal ceiling. 4 years so far....no heat...insulated well and o sign or rust.
If you spray foam insulation, no plastic sheet is needed
Musicinabottle
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The coating on the metal is superb at preventing rust. On homes, at least in my area which is cold and damp in the Winters, they for awhile put plastic on the walls, but never a ceiling, with the exception of those ceilings that had wood such as tongue and groove where moisture cold get through, but if it did get through, it could escape back through the wood. With drywall, they never put plastic on the ceiling because moisture vapor still gets through any penetrations such as light fixture boxes, hits the plastic then just sits there. So they apply a vapor barrier to the face of the drywall. With the metal on your ceiling, if you seal all the seams, then it's not possible for moisture vapor to get past.
You must put vapour barrier on a curling especialynin cold climate. This is house houses are built in Canada. Keep the warm noisy air from the house escaping into the attic to meet cold air and turn into water. Drywall, mud and paint are not a substitute. If you vapour barrier the warm side of the drywall you trap it inside and it will rot.
Radix2
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No need. There is zero vapor transmission through steel. If you use cellulose there will be very little airflow through the seams.
What is your garage use ? Any reason to think you will have high vapor things going on in there line hot showers, cooking, etc?
What is your garage use ? Any reason to think you will have high vapor things going on in there line hot showers, cooking, etc?
like2wheel
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This is how I always understood it to be, provided you have adequate roof ventilation.
Very interesting the difference in opinions. This is a workshop only (woodworking, vehicle mechanics) no reason for an excessive amount of moisture to be produced. The roof is vented, so it makes sense if any moisture did get accumulate it should evaporate. The walls are sprayfoamed, so it should be super tight. I decided to blow in the ceiling insulation for cost reasons. I was sort of thinking plastic would help seal the ceiling up too and keep the heat in during our cold winters. The difference in air temperature between the heated shop and cold attic was the reason I thought you always put plastic up.
Dick in Wisconsin
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I did the same thing. My shop is heated and ceiling is well insulated. Contractor put up poly vapor barrier. I think his goal was to keep any moisture out of the insulation above the steel.
Vapour barrier is a must. Tin does not seal vapour. It's just lapped and not that great most the time. You vapour the ceiling which stops the warm air from going through to the cold side of the insulation and creating condensation. This is how commercial and residential construction is done up here. I've been doing it for 15 years. You roof must be vented properly to allow air flow and remove any warm air as it will cause frost on the underside of the sheathing and it will rot as well. Trust me lol seen it lots
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I used the poly also.
You can see some pics and ideas in my build in my sig.
I did it because Morton buildings does it. If there was a real problem, chances are they wouldn't continue to use it.
Plus, If I ever want to pull a panel down, the insulation will stay put. I don't see any reason to, but you never know.
I'm doing mine by myself, I cant imagine trying to seal every joint on the tin with mastic!(or anything) It would be all over everything!
You can see some pics and ideas in my build in my sig.
I did it because Morton buildings does it. If there was a real problem, chances are they wouldn't continue to use it.
Plus, If I ever want to pull a panel down, the insulation will stay put. I don't see any reason to, but you never know.
I'm doing mine by myself, I cant imagine trying to seal every joint on the tin with mastic!(or anything) It would be all over everything!
Double J
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No vapor barrier in my sloped tin ceiling. My builder said it was not needed and that was good enough for me.
Musicinabottle
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The butyl bead comes in rolls. It's like clay or putty.
4cyclic
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Poly will help not only as vapour barrier but also reduce air infiltration. Do put it up.
Oh yeah, I have seen that stuff. That would probably be doable with two people like you said, if done right.
Musicinabottle
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They have all sorts of butyl bead, but the best stuff I've ever used, doing giant airplane hangers with a fully floating roof system capable of withstanding 130 MPH winds always smelled like electronics and was light gray.
sdwalter
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My building has spray foam on the walls and blown in insulation in the ceiling with poly vapor barrier too.
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ard
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You are getting good advice mixed with bad.
DO NOT necessarily listen to 'here is what I did so you should do the same'
DO listen to people that can provide a scientific/engineering basis for their recommendations.
Or go to a building website.
You think moisture will not be 'generated'? lol. It isnt about moisture being generated by specific tasks. Warm air holds more moisture than cooler. Period. Just you breathing in the shop WILL add moisture. Drive in a car that had been rained on hours ago- WILL add moisture. Drop off a load of green lumber to stabilize? Yup.
When that warm air some moisture in your shop leaks into the walls it will tend to condense that water out on cooler surfaces. This is a fact.
You can either control the thermal profile across the wall- OR- keep the moisture laden air away from the cooler parts of the wall.
A moisture barrier at or just behind the metal will still be in the 'warm' zone of that wall. You should not see condensation right behind a lightweight metal. That metal will basically be at the room temp.
Stapling up poly is WAY easier that sealing sheets of metal, around boxes, lights, against walls.
Try thinking of walls and ceilings as an assembly that is controlling water, air, moisture and heat. Go check out green building advisor (google it)
Dont just take my, or anyone's, word.
lakeroadster
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You are getting good advice mixed with bad.
DO NOT necessarily listen to 'here is what I did so you should do the same'
DO listen to people that can provide a scientific/engineering basis for their recommendations.
Or go to a building website.
You think moisture will not be 'generated'? lol. It isnt about moisture being generated by specific tasks. Warm air holds more moisture than cooler. Period. Just you breathing in the shop WILL add moisture. Drive in a car that had been rained on hours ago- WILL add moisture. Drop off a load of green lumber to stabilize? Yup.
When that warm air some moisture in your shop leaks into the walls it will tend to condense that water out on cooler surfaces. This is a fact.
You can either control the thermal profile across the wall- OR- keep the moisture laden air away from the cooler parts of the wall.
A moisture barrier at or just behind the metal will still be in the 'warm' zone of that wall. You should not see condensation right behind a lightweight metal. That metal will basically be at the room temp.
Stapling up poly is WAY easier that sealing sheets of metal, around boxes, lights, against walls.
Try thinking of walls and ceilings as an assembly that is controlling water, air, moisture and heat. Go check out green building advisor (google it)
Dont just take my, or anyone's, word.
Basically what I was saying lol but better words thanks. He poly is an envelope to separate 2 areas. You can also buy formed poly boxes for plugs and lights as opposed to cutting a square of poly and putting the box on top of it. Using red or blue tuck tape is easier and cleaner to join poly edges than using acoustical sealant. Butyl is not for poly laps as it doesn't stay playable like acoustical.
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ard
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Musicinabottle
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ard
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I am trying to get across that there is no "one way", but if you take the time to understand the WHY, when it comes to something unique or your project, you can make the right call.
(Reminds me of a guy here who had moisture issues with black mold growing on the unfinished wall cavities in his garage- he cleaned it, then painted with a mildewicide, and was planning on just covering it up. No real thought on vapor barriers, how to control air, etc. I was cringing.)
We are spending a lot of money on these projects, and a $0 mistake (mis-application, or wrong material) or omitting $50 of plastic can make or break the life of a structure.
493mike
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My shop ceiling is 29 gauge gloss white/lacquered back-clear. I did not use a vapor barrier (my beer does not leak out of the cans it comes in) but, blew in 12" of cellulose on top of the steel. I heat my shop all winter (60*) and have had no ill effects in over 17 years. I used 8" unfaced fiberglass batts in the walls behind the steel skin but, I did use plastic sheet for a barrier, in that situation, under the 5/8" sheet rock.
Mike
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Why do the walls of you don't do the ceiling? Your heat will escape from the ceiling? Your beer can is one piece not like a tin ceiling and it will come saw on the outside without being open won't it?
I would suggest that in the middle of winter you go and shovel some insulation off the back of your tin where air my be getting through say in corners or bad laps, and you will find moisture.
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I am trying to get across that there is no "one way", but if you take the time to understand the WHY, when it comes to something unique or your project, you can make the right call.
(Reminds me of a guy here who had moisture issues with black mold growing on the unfinished wall cavities in his garage- he cleaned it, then painted with a mildewicide, and was planning on just covering it up. No real thought on vapor barriers, how to control air, etc. I was cringing.)
We are spending a lot of money on these projects, and a $0 mistake (mis-application, or wrong material) or omitting $50 of plastic can make or break the life of a structure.
Heated building = vapour barrier. Unheated it's not needed because your not creating a temperature difference.
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R5P7Duster
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How about this scenario. I don't keep my shop heated all the time. It's 1000 square feet. In the winter it drops down to 20° inside, I heat it to 68° when I'm working in it. Walls are insulated just haven't gotten around to do the ceiling yet. So would you use plastic in this application as well?
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Adding heat will add moisture into the air. It will also increase the temperature difference that can cause condensation. You need to have some type of vapor barrier in place to keep that moisture from making it into the insulation above. So the vapor barrier used depends on the type of insulation you're planning. For cellulose insulation, plastic works great, but you can also use kraft faced fiberglass insulation as well since the facing acts as a vapor barrier.
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Musicinabottle
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Well, as with anything, opinions will vary, but in my area, they never put plastic on the ceilings. They used to plastic the walls back in the late 1980s for awhile, then they stopped. I've been through "Green Building" classes on advanced framing and insulation and other theories, and in short, your ceiling is much more subjected to heat loss and moisture vapor issues than are the walls. When it comes right down to it, in a garage or shop setting, you typically aren't going to have a whole lot of consistent moisture in the heated space like you would with a house. The only time I've ever used plastic on a ceiling is when you do a wood ceiling like T&G. If it's drywall, or metal, then you just don't see plastic used in my area anyway. I know you hear people in this thread stating that they have plastic behind the metal on their ceiling and never had moisture issues. That might be, but how do you really know? Have you set up a monitoring device between the metal and plastic? Has it been 35 years or so and still all good? I say this because in my house, that my father built, and I helped, we have a firewood storage area in the basement and my dad used plastic then metal and I hadn't noticed an issue until years later when little by little, we started getting water streaking down the walls. Not terrible, but definitely there. So this I believe is the reason why they don't plastic under metal on ceilings here anyway. I've since removed the metal and the plastic, and will be replacing it with FRP and caulking it, and we'll see over the course of the next 35 years if I have issues again.
My opinion on the moisture in your basement would be cause by the moisture not being able to escape. Ie cement wall wicking moisture in an no way to vent out the top.
A vapour barrier would be better served on ceiling than walls as it will retain the heat. The reason a work area that isn't heated regular or barely brought to above temperate wouldn't have much noticeable moisture is because it's escaping so fast it doesn't have time. Another reason to save money on heating with a vapour barrier. Like I said it's code up here in Canada. Where we see heat and cold often. It's also part of "green building" and LEED environmental building up here. Moisture is a beeyotch and I can say you need the vapour barrier but you also need adequate venting on the cold side of it.
Musicinabottle
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It's difficult to explain but this room, which is heated somewhat in the winter, is actually entirely wood framed, both walls and ceiling, with a dirt floor since we stack up to 12 cords of wood in there, so there is moisture for sure, but the surround area (walls and ceiling) are not in contact with dirt at all. The issue in my case, is that the moisture vapor charged air is getting past the metal and trapped against the plastic, and running down the rake of the ceiling to the wall. I once participated in the construction of a huge lumber kiln, and this is where first hand, I realized that you have to use sealant at your metal laps when you have extreme moisture. Also, another point to consider is that if you're heating your shop, then turning the heat off, then heating it again, and especially if you live in an area like I do where it can drop to -35 degrees, these hot cold cycles will cause condensation on your interior metal surfaces, both front and back, because if you have plastic, the plastic prevents the insulation, if blown in, from coming into contact with the back side ribs of your metal, so you have an air space that would allow moisture to collect. Probably not much, but when you don't use plastic, and instead seal all your joints, now your blown in can get down into the rib voids and prevent air space. Even if you did get moisture there, it could still escape up through the insulation, and eventually through your attic vents.